Methane fermentation method

ABSTRACT

A methane fermentation method having the steps of finely pulverizing an organic substance adjusted such that a moisture content of the organic substance is equal to or less than 10%, in a wet bead mill; supplying the organic substance which is finely pulverized in the pulverization step to a methane fermentation chamber and decomposing the organic substance with an anaerobic microorganism to cause methane fermentation so as to generate methane gas; separating, with a UF membrane, a digestive fluid after the methane fermentation in the methane fermentation step into a concentrated liquid and a permeation liquid; returning the concentrated liquid separated in the separation step to the methane fermentation chamber; and agitating the contents of the methane fermentation chamber with a water flow produced by the return of the concentrated liquid in the return step.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national phase application under 35 U.S.C. § 371 ofInternational Patent Application No. PCT/JP2015/071645, filed Jul. 30,2015, and claims benefit of priority to Japanese Patent Application No.2014-158980, filed Aug. 4, 2014. The entire contents of theseapplications are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a methane fermentation method and amethane fermentation system for generating methane gas by decomposingorganic substances with an anaerobic microorganism and thereby causingmethane fermentation.

BACKGROUND

Conventionally, for example, as a technology for generating methane gasby causing, with an anaerobic microorganism, methane fermentation onorganic substances containing a large amount of solid such as foodresidues, livestock manure and woody raw materials, a system is knownthat has a configuration in which the organic substances are pulverizedor crushed to be sieved into predetermined particle diameters and inwhich thereafter methane fermentation is caused (for example, seeJapanese Laid-open Patent Publication No. 2010-142735).

SUMMARY

However, in the configuration of PLT1 described above, specific meansfor pulverizing the organic substances and particle diameters after thepulverization are not provided, and even when organic substances arecrushed or pulverized with general crushing/pulverizing means, theefficiency of conversion into methane gas is low, with the result thatit is impossible to efficiently generate methane gas.

In particular, when organic substances containing a woody or fibrouscomponent are used, it takes much time to decompose the organicsubstances with an anaerobic microorganism, with the result that it isimpossible to efficiently generate methane gas.

The present invention is made in view of the foregoing points, and anobject thereof is to provide a methane fermentation method and a methanefermentation system with which it is possible to efficiently generatemethane gas.

An example methane fermentation method includes a pulverization step offinely pulverizing an organic substance in a wet bead mill; a methanefermentation step of supplying the organic substance which is finelypulverized in the pulverization step to a methane fermentation chamberand decomposing the organic substance with an anaerobic microorganism tocause methane fermentation so as to generate methane gas; a separationstep of separating, with a UF membrane, a digestive fluid after themethane fermentation in the methane fermentation step into aconcentrated liquid and a permeation liquid; a return step of returningthe concentrated liquid separated in the separation step to the methanefermentation chamber; and an agitation step of agitating contents of themethane fermentation chamber with a water flow produced by the return ofthe concentrated liquid in the return step.

An example methane fermentation method is configured such that in themethane fermentation method above, the methane fermentation step, theseparation step, the return step and the agitation step are repeated.

An example methane fermentation method is configured such that in thepulverization step, the organic substance is finely pulverized in thewet bead mill such that a particle diameter at 50% in a cumulativedistribution with respect to a volume is equal to or less than 20 μm.

An example methane fermentation method is configured such that in thepulverization step, a moisture content of the organic substance isadjusted such that a solid content is equal to or less than 10%, and theorganic substance is finely pulverized in the wet bead mill.

An example methane fermentation method is configured such that in thepulverization step, the organic substance is finely pulverized in thewet bead mill for one hour or more.

An example methane fermentation method is configured such that in theseparation step, the permeation liquid separated with the UF membrane isseparated with an RO membrane.

An example methane fermentation method is configured such that beforethe digestive fluid after the methane fermentation in the methanefermentation step is separated in the separation step, a colony of ananaerobic microorganism included in the digestive fluid is crushed.

An example methane fermentation method is configured such that thecolony of the anaerobic microorganism included in the digestive fluid iscrushed by mixing action of an inline mixer.

An example methane fermentation method is configured such that thecolony of the anaerobic microorganism included in the digestive fluid iscrushed by homogenizing action of a homogenization pump.

An example methane fermentation method is configured such that in thepulverization step, the organic substance is finely pulverized whilebeing cooled according to the anaerobic microorganism such that thefinely pulverized organic substance has a methane fermentationtemperature at which the anaerobic microorganism easily acts in themethane fermentation step, and in the methane fermentation step, theorganic substance having the methane fermentation temperature issupplied to the methane fermentation chamber.

An example methane fermentation method is configured such that thedigestive fluid from the methane fermentation step or the concentratedliquid from the separation step is heated by heat exchange action of aheat exchanger.

An example methane fermentation system which decomposes an organicsubstance with an anaerobic microorganism to cause methane fermentationso as to generate methane gas includes: a wet bead mill which finelypulverizes the organic substance; a methane fermentation chamber inwhich the organic substance finely pulverized in the wet bead mill isdecomposed with the anaerobic microorganism to cause the methanefermentation so as to generate the methane gas; a UF membrane separatorwhich concentrates and separates a digestive fluid after the methanefermentation in the methane fermentation chamber into a concentratedliquid and a permeation liquid; and return means which returns theconcentrated liquid to the methane fermentation chamber, where contentsof the methane fermentation chamber are agitated by the return of theconcentrated liquid with the return means.

An example methane fermentation system is configured such that the wetbead mill finely pulverizes the organic substance such that a particlediameter at 50% in a cumulative distribution with respect to a volume isequal to or less than 20 μm.

An example methane fermentation system is configured so as to include anRO membrane separator which separates the permeation liquid.

An example methane fermentation system is configured so as to includecrushing means which crushes a colony of the anaerobic microorganismincluded in the digestive fluid after the methane fermentation in themethane fermentation chamber.

An example methane fermentation system is configured such that thecrushing means is an inline mixer.

An example methane fermentation system is configured such that thecrushing means is a homogenization pump.

An example methane fermentation system is configured so as to includecooling means which cools the organic substance according to theanaerobic microorganism such that the organic substance finelypulverized in the wet bead mill has a methane fermentation temperatureat which the anaerobic microorganism easily acts in the methanefermentation chamber.

An example methane fermentation system is configured so as to include aheat exchanger which heats, by heat exchange action, the digestive fluidfrom the methane fermentation chamber or the concentrated liquid fromthe UF membrane separator.

According to the present invention, since the organic substance isfinely pulverized in the wet bead mill, the decomposition caused by theanaerobic microorganism easily proceeds, and thus the methane gas can beefficiently generated.

Moreover, since the organic substance finely pulverized in the wet beadmill is subjected to the methane fermentation, the pores of the UFmembrane are unlikely to be blocked by the digestive fluid, and thedigestive fluid is easily concentrated.

Furthermore, since the contents of the methane fermentation chamber areagitated with the water flow produced by the return of the concentratedliquid, the anaerobic microorganism uniformly acts on the finelypulverized organic substance, and thus the decomposition easilyproceeds.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustrative diagram showing the configuration of a methanefermentation system according to a first embodiment of the presentinvention;

FIG. 2 is an illustrative diagram showing the configuration of a methanefermentation system according to a second embodiment of the presentinvention;

FIG. 3 is an illustrative diagram showing the configuration of a methanefermentation system according to a third embodiment of the presentinvention;

FIG. 4 is a graph showing a particle size distribution in Example 1;

FIG. 5 is a graph showing a particle size distribution in Example 2;

FIG. 6 is a graph showing a particle size distribution in Example 3;

FIG. 7 is a graph showing the amount of methane generated in Examplesand Comparative Example;

FIG. 8 is a graph showing a methane conversion rate in the Examples andthe Comparative Example;

FIG. 9 is a graph showing the methane conversion rate in the Examplesand the Comparative Example when sorghum is used as the organicsubstance;

FIG. 10 is a graph showing the methane conversion rate in the Exampleswhen palm is used as the organic substance; and

FIG. 11 is a graph showing the methane conversion rate in the Examplesand the Comparative Example when coffee grounds are used as the organicsubstance.

DETAILED DESCRIPTION

The configuration of a first embodiment according to the presentinvention will be described in detail below with reference to FIG. 1.

In FIG. 1, reference sign 1 represents a methane fermentation system,and the methane fermentation system 1 decomposes an organic waste 2containing organic substances such as food residues, livestock manureand woody raw materials with a methane bacterium which is an anaerobicmicroorganism and thereby causes methane fermentation so as to generatemethane gas 3.

The methane fermentation system 1 includes: a wet bead mill 4 in whichthe organic waste 2 is finely pulverized; and a methane fermentationchamber 6 which is filled with the methane bacterium, in which thepulverized organic substance 5 that is the organic substances finelypulverized in the wet bead mill 4 are decomposed with the methanebacterium and in which thus methane fermentation is caused to generatethe methane gas 3. A UF membrane separator 8 and an RO membraneseparator 9 in which a digestive fluid 7 after the methane fermentationis concentrated and separated are connected to the methane fermentationchamber 6.

The wet bead mill 4 is conventionally used when for example, a metalsuch as a rare metal or a paint is finely pulverized, and a raw materialin a slurry state in which an item to be pulverized is mixed with aliquid is supplied per predetermined amount and beads which are putthereinto are used to perform the fine pulverization.

In the methane fermentation system 1, the organic waste 2 in a slurrystate mixed with the liquid is supplied to the wet bead mill 4 perpredetermined amount, and the organic waste 2 is finely pulverized intothe pulverized organic substance 5. In addition, depending on the typeand the state of the organic waste 2, preprocessing crushing means maybe separately provided in the preceding stage such that the organicwaste 2 is easily finely pulverized in the wet bead mill 4. In otherwords, a configuration may be adopted in which the organic waste 2 ispreviously crushed with the preprocessing crushing means and is mixedwith the liquid into a slurry state, and in which the organic waste 2 ina slurry state is finely pulverized in the wet bead mill 4.

An organic substance storage chamber 12 is connected through a pipingmember 11 to the wet bead mill 4, and the pulverized organic substance 5finely pulverized in the wet bead mill 4 is supplied through the pipingmember 11 to the organic substance storage chamber 12.

The organic substance storage chamber 12 is connected through a pipingmember 13 to the methane fermentation chamber 6, and the pulverizedorganic substance 5 stored in the organic substance storage chamber 12is supplied to the methane fermentation chamber 6 per predeterminedamount.

The methane fermentation chamber 6 is a sealed reaction chamber, and itsinterior is filled with the unillustrated methane bacterium and is keptunder anaerobic conditions. Within the methane fermentation chamber 6,an agitator for performing the mixing is installed such that the methanebacterium uniformly acts on the pulverized organic substance 5.

Then, within the methane fermentation chamber 6, the pulverized organicsubstance 5 is decomposed with the methane bacterium to cause methanefermentation, and as the methane fermentation proceeds, the methane gas3 and the digestive fluid 7 in which fertilizer components such asnitrogen and phosphorus are left by the decomposition of the methane aregenerated.

Specifically, it is highly likely that within the methane fermentationchamber 6, the methane gas generated by the methane fermentation remainsin the cavity portion of the uppermost portion, and that the digestivefluid 7 is stored in a portion below it.

As the methane fermentation proceeds, a higher concentration of themethane bacterium is generated toward a lower layer of the digestivefluid 7, and since the methane bacterium is adhered to the pulverizedorganic substance 5, a TS (evaporation residues) concentration isincreased. On the other hand, in an upper layer of the digestive fluid7, as the methane fermentation proceeds, the concentration of theorganic substances is lowered, and the TS concentration is lowered, withthe result that for example, the TS concentration becomes about 2 to 5%.

In other words, within the methane fermentation chamber 6, as themethane fermentation proceeds, the methane gas 3 remains in the cavityportion of the uppermost portion, and within the digestive fluid 7 inwhich the fertilizer components are left, the upper layer whose TSconcentration is low and the lower layer whose TS concentration is highare present. In addition, even in the upper layer of the digestive fluidwhose TS concentration is low, the TS such as the pulverized organicsubstance 5 which is not decomposed with the methane bacterium and whichis not digested is contained.

The UF membrane separator 8 is connected through a piping member 14 tothe methane fermentation chamber 6, and the upper layer of the digestivefluid 7 within the methane fermentation chamber 6 is supplied to the UFmembrane separator 8.

In the UF membrane separator 8, a UF membrane (ultrafiltration membrane)which includes pores having, for example, a diameter of about 0.03 μm isused, and thus the digestive fluid 7 is separated into a concentratedliquid 15 that contains the TS such as the pulverized organic substance5 which does not pass the pores and which is formed in the shape of fineparticles and a permeation liquid 16 which passes the pores.

The RO membrane separator 9 is connected through a piping member 17 tothe UF membrane separator 8, and the permeation liquid 16 is suppliedfrom the UF membrane separator 8 to the RO membrane separator 9.

Also, the methane fermentation chamber 6 is connected to the UF membraneseparator 8 through a piping member 18 which is a return path and whichserves as return means, and the concentrated liquid 15 is returned fromthe UF membrane separator 8 to the methane fermentation chamber 6.

Then, the concentrated liquid 15 is returned from the UF membraneseparator 8 to the methane fermentation chamber 6, and thus a water flowis produced within the methane fermentation chamber 6, with the resultthat the contents of the methane fermentation chamber 6 are agitated bythis water flow.

In addition, although it is likely that the digestive fluid 7 within themethane fermentation chamber 6 is not clearly separated into the upperlayer whose TS concentration is low and the lower layer whose TSconcentration is high, the piping member 14 and the piping member 18serving as circulation lines between the methane fermentation chamber 6and the UF membrane separator 8 are constantly kept in an anaerobicstate, and thus the UF membrane is not blocked even in a state where theupper layer and the lower layer of the digestive fluid 7 are partiallymixed, with the result that the action of enhancing or maintaining theoverall TS concentration within the methane fermentation chamber 6caused by returning the concentrated liquid 15 is prevented from beinglowered.

Since the concentrated liquid 15 is returned to the methane fermentationchamber 6, the methane fermentation in the methane fermentation chamber6, the concentration and separation of the digestive fluid 7 in the UFmembrane separator 8, the return of the concentrated liquid 15 from theUF membrane separator 8 to the methane fermentation chamber 6 and theagitation of the contents of the methane fermentation chamber 6 causedby the return are substantially repeated.

The RO membrane separator 9 separates, with an RO membrane (reverseosmosis membrane), the permeation liquid 16 on an ion level. In otherwords, even the permeation liquid 16 which is separated from theconcentrated liquid 15 in the UF membrane separator 8 contains ionizedcomponents and thus cannot be drained without being processed withconsideration given to drainage standards and the like. Hence, in the ROmembrane separator 9, the permeation liquid 16 is separated intoconcentrated water 21 containing the fertilizer components andpermeation water 22 which can be drained.

Then, the discharged amount of concentrated water 21 obtained in the ROmembrane separator 9 can be reduced to 30% or less of the input amountof organic waste 2 and can be reused as a liquid fertilizer whoseconcentration is higher than normal.

Also, the permeation water 22 obtained in the RO membrane separator 9can be processed, as necessary, such as by being reused in anapplication such as for diluting the organic substances or the like orby being drained.

A methane fermentation method in the methane fermentation system 1described above will then be described.

When the organic waste 2 serving as organic substance is decomposed withthe methane bacterium and is subjected to methane fermentation so as togenerate the methane gas 3, the organic waste 2 is first finelypulverized in the wet bead mill 4.

When the organic waste 2 is finely pulverized, the organic waste 2 andwater are mixed with each other so as to be brought into a slurry state.In addition, when the preprocessing crushing means is installed in thepreceding stage, the organic waste 2 may be brought into a slurry stateby being crushed with the preprocessing crushing means and also beingmixed with water.

Here, in order for the organic waste 2 to be pulverized in the wet beadmill 4, the moisture content of the slurry is very important.Specifically, for example, when a metal such as a rare metal isconventionally pulverized in the wet bead mill 4, the moisture contentis adjusted such that the slurry contains a solid content of 15 to 30%.However, since the specific gravity of the organic waste 2 is less than2, is less than that of the metal and is less than that of water, isslightly less than that of water or is equal to that of water, theproceeding state of the pulverization in the wet bead mill 4 differsfrom the metal. In other words, since the organic waste 2 contains anamino acid, lignin, a protein and the like, as the pulverizationproceeds, the viscosity thereof is increased. When the viscosity isexcessively increased, the circulation of the organic waste 2 within thewet bead mill 4 is inhibited, and thus it is difficult for thepulverization to proceed, with the result that it is impossible topulverize the organic waste 2 into a predetermined size.

Hence, preferably, when the organic waste 2 is pulverized in the wetbead mill 4, the moisture is adjusted such that the solid content in theslurry is equal to or less than 10% (TS).

The organic waste 2 in a slurry state whose moisture is adjusted asdescribed above is supplied per predetermined amount to the wet beadmill 4 and is finely pulverized.

Since the solid content in the organic waste 2 in a slurry state isequal to or less than 10%, in the early stage of the pulverization, theviscosity of the organic waste 2 in a slurry state in the process of thepulverization is not achieved, and when in such a state, the bead mill 4is rotated at a high speed, the organic waste 2 cannot be appropriatelypulverized, and the size of a center vortex may be increased such thatan overflow occurs.

Hence, for 30 minutes after the start of the pulverization, thepulverization is performed by rotating the bead mill at a medium speed,and after 30 minutes have elapsed since the start of the pulverizationand a certain degree of viscosity is achieved, the pulverization isperformed by rotating the bead mill at the maximum speed for up to twohours.

As described above, the bead mill is rotated at a medium speed in theearly stage of the pulverization, then the bead mill is rotated at ahigh speed and thus it is possible to appropriately and finely pulverizethe organic waste 2 such that for example, the particle diameter (mediandiameter) at 50% in a cumulative distribution (accumulated distribution)with respect to the volume is equal to or less than 20 μm. In addition,it is more preferable to pulverize the organic waste 2 such that themedian diameter of the organic waste 2 is equal to or more than 0.8 μmand equal to or less than 10 μm.

The pulverized organic substance 5 which is finely pulverized istemporarily stored in the organic substance storage chamber 12, and issupplied per predetermined amount to the methane fermentation chamber 6.

Then, the pulverized organic substance 5 is decomposed with the methanebacterium within the methane fermentation chamber 6 so as to causemethane fermentation, and thus the methane gas 3 is generated and thedigestive fluid 7 in which the other fertilizer components such asnitrogen and phosphorus are left by the decomposition of the methane isgenerated.

The methane gas 3 generated by the methane fermentation is collected asnecessary and is utilized as, for example, an energy source for anengine generator.

In the digestive fluid 7 generated by the methane fermentation, the TSconcentration of the upper layer is relatively low, the TS concentrationof the lower layer is relatively high and the methane fermentationproceeds in the lower layer. Hence, the upper layer whose TSconcentration is low is supplied to the UF membrane separator 8.

In the UF membrane separator 8, the digestive fluid 7 is separated, withthe UF membrane, into the concentrated liquid 15 containing the TS andthe permeation liquid 16 passing the pores of the UF membrane. Inaddition, since bacteria, germs and the like cannot pass the pores (forexample, a diameter of about 0.03 μm) of the UF membrane, even when themethane bacterium is contained in the digestive fluid 7, the methanebacterium is separated together with the concentrated liquid 15.

Since the concentrated liquid 15 separated in the UF membrane separator8 in this way has, as the main component, TS components containing themethane bacterium, the concentrated liquid 15 is returned to the methanefermentation chamber 6 and is utilized for reducing a decrease in the TSconcentration within the methane fermentation chamber 6 whereas thepermeation liquid 16 containing the fertilizer components which containalmost no TS components and which are ionized is supplied to the ROmembrane separator 9.

In addition, the concentrated liquid 15 is returned to the methanefermentation chamber 6, and thus the water flow is produced within themethane fermentation chamber 6, with the result that the contents of themethane fermentation chamber 6 are agitated by this water flow.

In the RO membrane separator 9, the permeation liquid 16 is separated,with the RO membrane, into the concentrated water 21 containing thefertilizer components and the permeation water 22 which can be drained.

Then, the concentrated water 21 is reused as a liquid fertilizer, andthe permeation water 22 is reused as dilution water or is drained.

The action and effects of the first embodiment discussed above will thenbe described.

In the methane fermentation system 1 and the methane fermentation methoddescribed above, even when the organic waste 2 contains an organiccomponent such as a woody or fibrous component in which it is difficultfor the decomposition with the methane bacterium to proceed, since theorganic waste 2 is finely pulverized in the wet bead mill 4 into such asize that it is impossible to pulverize the organic waste 2 into thesize with a conventional crusher or the like, the decomposition with themethane bacterium easily proceeds, with the result that it is possibleto enhance a methane conversion rate, to reduce a decomposition time andto efficiently generate the methane gas.

In particular, the organic waste 2 is finely pulverized in the wet beadmill 4 such that the particle diameter at 50% in the cumulativedistribution with respect to the volume is equal to or less than 20 μm,and thus the decomposition of the organic waste 2 with the methanebacterium easily proceeds, with the result that it is possible to moreefficiently generate the methane gas.

Also, when the organic waste 2 is crushed with, for example, a crusheras conventionally performed, since the particle diameter of theundigested TS contained in the digestive fluid 7 is large with respectto the pore of the UF membrane, the pores are blocked when the digestivefluid 7 is separated with the membrane in the UF membrane separator 8.Specifically, when the digestive fluid 7 to be separated contains just afew percent of a substance having a diameter of 4 mm or more, the poresof the UF membrane is blocked.

However, as described above, the organic waste 2 is finely pulverized inthe wet bead mill 4 such that the median diameter is equal to or lessthan 20 μm, and thus the pores of the UF membrane are seldom blocked bythe undigested pulverized organic substance 5 contained in the digestivefluid 7.

In other words, the organic waste 2 is finely pulverized such that themedian diameter is equal to or less than 20 μm, and thus when thedigestive fluid 7 after the methane fermentation is concentrated in theUF membrane separator 8, the pores of the UF membrane are unlikely to beblocked by the undigested pulverized organic substance 5 left in thedigestive fluid 7, with the result that it is possible to smoothlyconcentrate the digestive fluid 7.

When the organic waste 2 is finely pulverized in the wet bead mill 4,the moisture content of the organic waste 2 is adjusted such that thesolid content is equal to or less than 10%, and thus it is possible toprevent the viscosity from being excessively increased in the process ofthe pulverization, with the result that it is possible to finelypulverize the organic waste 2 into a desired particle size, for example,a median diameter of 20 μm or less.

Here, for example, since when membrane separation is performed, themembrane separation is normally started in a state where a moisturecontent is previously reduced, the membrane separation is stepwiseperformed so as not to block the membrane. Then, when the moisturecontent is normally reduced so as to increase the solid of the organicwaste 2 in a slurry state, and the concentration is performed in the UFmembrane separator 8 after the pulverization and the methanefermentation, the pores of the UF membrane are highly likely to beblocked. Hence, the moisture content of the organic waste 2 in a slurrystate is adjusted before the methane fermentation (before the finepulverization) such that the solid content is equal to or less than 10%,and thus it is possible to acquire the fluidity of the digestive fluid 7and to prevent the UF membrane from being blocked by the fact that whenthe digestive fluid 7 is concentrated, the concentration of the solid inthe digestive fluid 7 is increased and thus the fluidity is lowered,with the result that the digestive fluid 7 is easily concentrated.

Hence, the moisture content of the organic waste 2 is adjusted, and thusit is possible to appropriately finely pulverize the organic waste 2, tomake the methane fermentation easily proceed and to easily concentratethe digestive fluid 7 after the methane fermentation, with the resultthat a series of steps from the methane fermentation to theconcentration and separation can be made to proceed efficiently.

The digestive fluid 7 after the methane fermentation is concentrated andseparated with the UF membrane, and thus the TS such as the pulverizedorganic substance 5 contained in the digestive fluid 7 can beconcentrated and separated from the digestive fluid 7, and theconcentrated liquid 15 containing the TS is returned to the methanefermentation chamber 6, and thus it is possible to perform the methanefermentation without waste and efficiently.

Also, the concentrated liquid 15 is returned from the UF membraneseparator 8 to the methane fermentation chamber 6, and thus the waterflow is produced within the methane fermentation chamber 6 so as to havethe action of agitating the contents of the methane fermentation chamber6, with the result that the power of the agitator installed within themethane fermentation chamber 6 can be reduced.

Furthermore, the digestive fluid 7 contains a large number of ionizedcomponents, and among them, ammonia is a component which inhibits themethane fermentation when the concentration thereof is increased withinthe methane fermentation chamber 6 but the digestive fluid 7 isseparated with the UF membrane and thus the ionized components can beseparated from the digestive fluid 7 and the concentrated liquid 15,with the result that it is possible to reduce the risk of inhibiting themethane fermentation.

The permeation liquid 16 after the concentration and separation with theUF membrane is separated with the RO membrane, and thus the permeationliquid 16 can be purified enough to be drained and can be processedeasily and appropriately. In other words, since the ionized componentscontained in the permeation liquid 16 can be separated with the ROmembrane as the concentrated water 21, the concentrated water 21containing the fertilizer components such as ammonia nitrogen,phosphorus, potassium and the like which are ionized can be effectivelyutilized as a concentrated liquid fertilizer or the like, and thepermeation water 22 can be reused or be drained without being processed.

Although it can be considered that the amount of low-concentrationliquid fertilizer obtained by the methane fermentation is significant,and that thus it is disadvantageously difficult to utilize it withconsideration given to a transportation method, an operation method andthe like, the concentration and separation are performed with the UFmembrane and the RO membrane, and thus it is possible not only to obtaina high-concentration liquid fertilizer as the concentrated water 21 butalso to reduce the volume to, for example, a fifteenth to a tenth ascompared with a conventional concentrating/separating method, with theresult that it is possible to cope with the problems in thetransportation and the operation.

In addition, although in the first embodiment described above, as theraw material, the organic waste 2 is used, there is no limitation tosuch a configuration, and as long as an organic substance is contained,a raw material which is not a waste can be applied.

Also, although the digestive fluid 7 after the methane fermentation isconcentrated and separated with the UF membrane, and the concentratedand separated permeation liquid 16 is concentrated and separated withthe RO membrane, there is no limitation to such a configuration, and thepermeation liquid 16 may be processed such as by being concentrated andseparated in another method.

A second embodiment will then be described with reference to FIG. 2. Thesame configurations and action as in the first embodiment are identifiedwith the same reference signs, and the description thereof will beomitted.

As shown in FIG. 2, as crushing means for crushing the colony of methanebacteria contained in a digestive fluid 7 a after methane fermentation,an inline mixer 32 is connected to a methane fermentation chamber 6 in amethane fermentation system 31, and a UF membrane separator 8 and an ROmembrane separator 9 in which the digestive fluid 7 b after the colonyis crushed is concentrated and separated are connected to the downstreamside of the inline mixer 32.

Here, in the methane fermentation chamber 6, in order for the methanefermentation to proceed, an organic substance raw material, afermentation temperature, pH, a small amount of inorganic salt metal anda fermentation inhibiting factor are controlled, and thus the balancebetween the growth and death of the methane bacteria is held, with theresult that the growth and death are managed to be in an equilibriumstate.

In the course of the growth of the methane bacteria, the methanebacteria form a colony (bacterial colony) which is an aggregation of thegrown bacteria.

Also, since the methane bacterium itself is significantly lightweight,the colony which is an aggregation of the methane bacteria is notprecipitated within the methane fermentation chamber 6, and in thedigestive fluid 7 a, the colony is floated in the shape of, for example,a biofilm (biological film) or forms an upper layer surface of thedigestive fluid 7 a.

The colony of the methane bacteria described above may block the UFmembrane of the UF membrane separator 8. In other words, when thedigestive fluid 7 a containing the colony of the methane bacteria isconcentrated and separated in the UF membrane separator 8, the pores ofthe UF membrane may be clogged by the colony and thus the UF membranemay be blocked.

Hence, in a piping member 14 between the methane fermentation chamber 6and the UF membrane separator 8, the inline mixer 32 is provided, andthe digestive fluid 7 a passes the inline mixer 32, with the result thatthe colony in the digestive fluid 7 a is crushed and that the methanebacteria are broken apart. In other words, with a shear force based onthe mixing action of the inline mixer 32 when the digestive fluid 7 apasses the inline mixer 32, the colony in the digestive fluid 7 a iscrushed.

The digestive fluid 7 a passes the inline mixer 32, thus the colony iscrushed and a digestive fluid 7 b whose TS concentration is low issupplied to the UF membrane separator 8.

Also, a concentrated liquid 15 after the concentration and separationwith the UF membrane separator 8 is returned to the methane fermentationchamber 6, and thus the methane fermentation in the methane fermentationchamber 6, the crushing of the colony in the digestive fluid 7 a withthe inline mixer 32, the concentration and separation of the digestivefluid 7 b in the UF membrane separator 8, the return of the concentratedliquid 15 from the UF membrane separator 8 to the methane fermentationchamber 6 and the agitation of the contents of the methane fermentationchamber 6 are substantially repeated.

In the methane fermentation system 31 described above, the colonyincluded within the digestive fluid 7 a from the methane fermentationchamber 6 is crushed with the inline mixer 32, and thus the methanebacteria are broken apart, with the result that it is possible toprevent the pores of the UF membrane from being blocked (clogged) by thecolony. Hence, it is possible to appropriately concentrate and separatethe digestive fluid 7 b with the UF membrane separator 8 and to make themethane fermentation proceed efficiently by returning the concentratedliquid 15 to the methane fermentation chamber 6.

Also, after the colony of the methane bacteria in the digestive fluid 7a from the methane fermentation chamber 6 is crushed, the concentratedliquid 15 concentrated in the UF membrane separator 8 is returned to themethane fermentation chamber 6 and is circulated, and thus as comparedwith a case where within the methane fermentation chamber 6, the methanebacteria continue to be present as the colony, it is possible toincrease the surface area of the methane bacteria. Hence, the digestingaction and the growing action of the pulverized organic substance 5 ofthe methane bacteria in the methane fermentation chamber 6 can befacilitated, with the result that it is possible to enhance theefficiency of the methane fermentation.

Since the inline mixer 32 can be provided in the piping member 14between the methane fermentation chamber 6 and the UF membrane separator8, the inline mixer 32 can be easily installed, and a mixing tank or thelike is not needed, with the result that the facilities are preventedfrom being enlarged.

Also, since the organic waste 2 is finely pulverized in the wet beadmill 4 such that the median diameter is equal to or less than 20 μm, andthe colony included in the digestive fluid 7 a is crushed with theinline mixer 32, the UF membrane separator 8 is unlikely to be blockedby the organic waste 2 and the colony, with the result that the smallpulverized organic substance 5 and the broken methane bacteria easilyact. In other words, by synergy of the configuration in which theorganic waste 2 is finely pulverized and the configuration in which thecolony included in the digestive fluid 7 a is crushed, it is possible tomake the methane fermentation in the methane fermentation chamber 6 andthe concentration and separation in the UF membrane separator 8 smoothlyproceed. Hence, it is possible to make a series of steps from themethane fermentation to the concentration and separation efficientlyproceed, and it is also possible to enhance the efficiency of themethane fermentation by returning and circulating the concentratedliquid 15 from the UF membrane separator 8 to the methane fermentationchamber 6.

In addition, although in the second embodiment described above, theinline mixer 32 is used as the crushing means, there is no limitation tosuch a configuration, and as long as the crushing means can crush thecolony in the digestive fluid 7 a, any configuration may be adopted.

Specifically, for example, as the crushing means, a homogenization pump(homogenizer) may be used.

In the configuration in which the homogenization pump is used as thecrushing means, by the homogenizing action of the homogenization pump,the synergy of shear, crash, cavitation and the like is instantaneouslyproduced, and thus the colony in the digestive fluid 7 a is crushed.

Then, the homogenization pump can be utilized not only as the crushingmeans for crushing the colony but also as a drive source for supplyingthe digestive fluids 7 a and 7 b from the methane fermentation chamber 6to the UF membrane separator 8, and thus it is possible to simplify thefacilities.

A third embodiment will then be described with reference to FIG. 3. Thesame configurations and action as in the first embodiment are identifiedwith the same reference signs, and the description thereof will beomitted.

As shown in FIG. 3, a UF membrane separator 8 and an RO membraneseparator 9 are connected to a methane fermentation chamber 36 in amethane fermentation system 35 through a heat exchanger 37 for heating adigestive fluid 7.

Here, a methane bacterium differs depending on the type thereof in atemperature suitable for the synthesis of methane (methane fermentationtemperature). For example, in the case of a so-called medium-temperaturefermentation methane bacterium, a temperature of about 38° C. issuitable for the synthesis of methane whereas in the case of a so-calledhigh-temperature fermentation methane bacterium, a temperature of about55° C. is suitable for the synthesis of methane.

In other words, in order to make the methane fermentation appropriatelyproceed in the methane fermentation chamber 36, it is important to keepthe interior of the methane fermentation chamber 36 at the methanefermentation temperature at which the methane bacterium easily acts.

Hence, by the heat exchange action of the heat exchanger 37 which isprovided between the methane fermentation chamber 36 and the UF membraneseparator 8 through a pump 38 serving as a power source for circulation,a digestive fluid 7 is heated to a predetermined temperature.

Also, since the heat exchanger 37 is provided in a circulation path fromthe methane fermentation chamber 36 through the UF membrane separator 8,the supply of the digestive fluid 7 from the methane fermentationchamber 36 to the heat exchanger 37, the supply of the digestive fluid 7from the heat exchanger 37 to the UF membrane separator 8 and the returnof a concentrated liquid 15 from the UF membrane separator 8 to themethane fermentation chamber 36 are performed by drive of the one pump38.

Then, the digestive fluid 7 from the methane fermentation chamber 36 isheated by the heat exchanger 37 and is supplied to the UF membraneseparator 8, and the concentrated liquid 15 from the UF membraneseparator 8 is returned to the methane fermentation chamber 36 and iscirculated.

Also, the digestive fluid 7 is heated and circulated as described above,and thus a substantially total amount of contents of the methanefermentation chamber 36 is heated, with the result that the interior ofthe methane fermentation chamber 36 is heated to the methanefermentation temperature. In other words, by the return of theconcentrated liquid 15 after the concentration and separation in the UFmembrane separator 8, the heat exchanger 37 is controlled to heat thedigestive fluid 7 such that the interior of the methane fermentationchamber 36 becomes the methane fermentation temperature.

Also, in order for the temperature of the interior of the methanefermentation chamber 36 not to be easily changed by a pulverized organicsubstance 5 supplied, in a wet bead mill 39, the pulverized organicsubstance 5 is finely pulverized while being cooled according to themethane bacterium within the methane fermentation chamber 36 such thatthe pulverized organic substance 5 finely pulverized has the methanefermentation temperature at which the methane bacterium easily actswithin the methane fermentation chamber 36.

Specifically, the temperature is managed by cooling the tank of the wetbead mill 39 to which the organic waste 2 is supplied such that thetemperature of the finely pulverized substance within the tank becomesthe methane fermentation temperature. For example, in the case of themedium-temperature fermentation methane bacterium, the cooling of thetank is controlled such that the temperature of the finely pulverizedsubstance within the tank of the wet bead mill 39 becomes about 38° C.whereas in the case of the high-temperature fermentation methanebacterium, the cooling of the tank is controlled such that thetemperature of the finely pulverized substance within the tank of thewet bead mill 39 becomes about 55° C.

An organic substance storage chamber 12 is connected to the wet beadmill 39 through a piping member 11, and the pulverized organic substance5 which is finely pulverized in the wet bead mill 39 is supplied throughthe piping member 11 to the organic substance storage chamber 12.

The organic substance storage chamber 12 has a heat retention structurecapable of retaining the temperature of the contents, and the pulverizedorganic substance 5 which is finely pulverized while being cooled to themethane fermentation temperature in the wet bead mill 39 is stored inthe organic substance storage chamber 12 in a state where thetemperature of the pulverized organic substance 5 is retained.

The organic substance storage chamber 12 is connected through a pipingmember 13 to the methane fermentation chamber 36, and the pulverizedorganic substance 5 stored in the organic substance storage chamber 12in the temperature-retained state is supplied to the methanefermentation chamber 36 per predetermined amount.

Then, in the methane fermentation system 35 described above, since theorganic waste 2 is finely pulverized while being cooled to the methanefermentation temperature in the wet bead mill 39, a change in thetemperature within the methane fermentation chamber 36 caused by thesupply of the pulverized organic substance 5 can be reduced by theutilization of thermal energy generated by the fine pulverization of theorganic waste 2. Hence, the thermal energy generated by the finepulverization is utilized for managing the temperature within themethane fermentation chamber 36, and thus the energy for heating theinterior of the methane fermentation chamber 36 to the methanefermentation temperature is reduced, with the result that it is possibleto make the methane fermentation proceed efficiently.

Also, since the wet bead mill 39 is cooled according to the methanebacterium of the methane fermentation chamber 36 such that the finelypulverized organic waste 2 has the methane fermentation temperature, themethane bacterium within the methane fermentation chamber 36 isprevented from being inactivated by the heat of the pulverized organicsubstance 5 supplied to the methane fermentation chamber 36, and thus itis possible to make the methane fermentation with the methane bacteriumappropriately proceed.

The digestive fluid 7 after the methane fermentation in the methanefermentation chamber 36 is heated to a predetermined temperature by theheat exchange action of the heat exchanger 37 provided between themethane fermentation chamber 36 and the UF membrane separator 8 in thecirculation path and is then supplied to the UF membrane separator 8,with the result that the one pump 38 can serve both as a power sourcefor supplying the digestive fluid 7 to the heat exchanger 37 and therebyheating it and as a power source for supplying the digestive fluid 7 tothe UF membrane separator 8. In other words, the heat exchanger 37 forheating the interior of the methane fermentation chamber 36 is providedin the circulation path, and thus the power source for the heating doesnot need to be provided separately of the power source for thecirculation (the concentration and separation and the return), with theresult that the power cost is reduced and that it is possible to makethe methane fermentation proceed efficiently.

In addition, for example, when a configuration is adopted in which meansfor heating a heat pipe and the like is incorporated in the methanefermentation chamber 36 itself in order to heat the interior of themethane fermentation chamber 36 to the methane fermentation temperature,it is likely that scale is produced on the outer surface and the innersurface of the heat pipe. It is very difficult to perform a maintenanceoperation of, for example, removing such scale and replacing the heatpipe in the configuration in which the heat pipe and the like areprovided in the methane fermentation chamber 36 itself, and thus it maybe impossible to appropriately manage the temperature within the methanefermentation chamber 36 by stopping the heating of the interior of themethane fermentation chamber 36, with the result that it may beimpossible to make the methane fermentation efficiently proceed.

Hence, the heat exchanger 37 which is the means for heating the interiorof the methane fermentation chamber 36 to the methane fermentationtemperature is provided outside (between the methane fermentationchamber 36 and the UF membrane separator 8) the methane fermentationchamber 36, and thus it is possible to avoid the risk on themaintenance, the replacement and the like, and it is also possible toefficiently perform the operation by the utilization of the power sourcefor the circulation as described above.

In addition, although the configuration in which as in the thirdembodiment described above, the heat exchanger 37 for heating theinterior of the methane fermentation chamber 36 to the methanefermentation temperature is provided in the circulation path ispreferable because the one power source can serve both as the powersource for the heating and the power source for the circulation, thereis no limitation to such a configuration, and the configuration and theinstallation position of the heating means for heating the interior ofthe methane fermentation chamber 36 to the methane fermentationtemperature can be determined as necessary.

Also, in the case of the configuration in which the heat exchanger 37 isprovided in the circulation path, there is no limitation to theconfiguration in which the digestive fluid 7 from the methanefermentation chamber 36 is heated between the methane fermentation stepand the separation step, and a configuration may be adopted in which theconcentrated liquid 15 is heated between the separation step and thereturn step.

In addition, a configuration obtained by combining the second embodimentand the third embodiment described above may be adopted. In other words,for example, between the pump 38 and the heat exchanger 37 shown in FIG.3, the inline mixer 32 which is the crushing means may be provided orinstead of the pump 38 shown in FIG. 3, the homogenization pump which isthe crushing means may be provided.

Examples

Examples and comparative example will be described below.

The efficiency of methane fermentation caused by a difference in theparticle size of an organic substance was first checked.

An organic substance whose moisture content was adjusted so as to have aslurry state where a solid was about 7% was pulverized in a wet beadmill for one hour, and the pulverized sample was used for Example 1. Theparticle size distribution of the sample in Example 1 is shown in FIG.4.

In Example 1, the median diameter which was a particle diameter at 50%in a cumulative distribution with respect to the volume was 19.7 μm, andthe arithmetic standard deviation was 31.1 μm.

An organic substance whose moisture content was adjusted as in Example 1was pulverized in the wet bead mill for one and a half hours, and thepulverized sample was used for Example 2. The particle size distributionof the sample in Example 2 is shown in FIG. 5.

In Example 2, the median diameter was 14.4 μm, and the arithmeticstandard deviation was 17.4 μm.

An organic substance whose moisture content was adjusted as in Examples1 and 2 was pulverized in the wet bead mill for two hours, and thepulverized sample was used for Example 3. The particle size distributionof the sample in Example 3 is shown in FIG. 6.

In Example 3, the median diameter was 11.9 μm, and the arithmeticstandard deviation was 12.6 μm.

Also, an organic substance was crushed, as in a conventional manner,with a crushing mixer so as to have a particle diameter of 1 to 5 mm,and the crushed sample was used for Comparative Example.

In these Examples and Comparative Example, the same methane bacteriumwas used, methane fermentation was performed under the same conditionsand the amount of methane generated and a methane conversion rate weremeasured.

In the Examples and Comparative Example, the averages of the amounts ofmethane generated in three specimens were shown in FIG. 7, and theaverages of the methane conversion rates in the three specimens wereshown in FIG. 8.

As shown in FIGS. 7 and 8, in all of examples 1 to 3 where the organicsubstance was finely pulverized in the wet bead mill, as compared withthe Comparative Example where the organic substance was processed withthe crushing mixer, the amount of methane generated and the methaneconversion rate were satisfactory, differences were produced immediatelyafter the start of the methane fermentation and the proceeding speed ofthe methane fermentation clearly differed.

When examples 1 to 3 are compared with each other, as the time duringwhich the organic substance was pulverized in the wet bead mill waslonger, and the particle diameter was smaller, the amount of methanegenerated, the methane conversion rate and the proceeding speed of themethane fermentation were more satisfactory.

Then, as the organic substance, sorghum, palm or coffee grounds wereused, and in order for the efficiency of methane fermentation caused bya difference in the particle size of the organic substances after beingpulverized to be checked, the amounts of individual samples were alignedwith COD values, and a BMP (Biochemical Methane Production) test wasperformed according to ISO14853.

The samples which were obtained by finely pulverizing the organicsubstances in the wet bead mill to have a particle diameter of 2 to 10μm were assumed to be nano-processed products (Examples), the sampleswhich were likewise obtained by finely pulverizing the organicsubstances to have a particle diameter of 100 to 500 μm were assumed tobe micro-processed products (Examples) and the sample which was obtainedby processing, with the crushing mixer, the organic substance to have aparticle diameter of 1 to 5 mm was assumed to be a mixer-processedproduct (Comparative Example).

The micro-processed products and the nano-processed products in theExamples and the mixer-processed product in the Comparative Example weredried at 110° C. for two hours, thereafter the weights thereof weremeasured and the TS thereof were determined.

Also, for the micro-processed products and the nano-processed productsin the Examples, 20 μl was weighed and was put into a vial for CODreaction (100-fold dilution) together with 1980 μl of water and the CODthereof was measured.

For the mixer-processed product in the Comparative Example, 2.0 mg wasput into the vial for COD reaction together with 2000 μl of water, andthe COD thereof was measured with a measuring device of HACH Company.

In the BMP test, the vial bottle (final volume of 50 ml) was sealed, wasthen subjected to nitrogen substitution and was placed within a constanttemperature oven of 55° C.

Then, a biogas which was generated over time was collected, thegenerated amount was measured, methane gas and CO₂ components weremeasured by gas chromatography and the methane conversion rate wascalculated.

Each of the measurements described above was performed on threespecimens in each of the samples. The results of the measurements areshown in Table 1. The transition of the methane conversion rate in eachof the samples is shown in FIG. 9.

TABLE 1 Amount Input Input Input CH4max Sample Sludge Water of samplegCOD/TS COD (mg/l) TS (%) amount (g) amount (ml) COD (g) (ml)Mixer-processed 1 80 g  20 g 0.2 g  1.28 — 91.45 0.22 — 0.2572 98.27Mixer-processed 2 1.28 — 91.45 0.22 — 0.2572 98.27 Mixer-processed 31.28 — 91.45 0.22 — 0.2572 98.27 Micro-processed 1 80 g 17.7 g 2.3 ml —77,700 — — 3.4 0.2642 100.93 Micro-processed 2 — 77,700 — — 3.4 0.2642100.93 Micro-processed 3 — 77,700 — — 3.4 0.2642 100.93 Nano-processed 180 g 15.3 g 4.7 ml — 51,600 — — 5.0 0.2580 98.57 Nano-processed 2 —51,600 — — 5.0 0.2580 98.57 Nano-processed 3 — 51,600 — — 5.0 0.258098.57

When the sorghum was used as the organic substance, as shown in Table 1,methane gas was generated in each of the nano-processing, themicro-processing and the mixer-processing.

As shown in FIG. 9, when the nano-processing was performed, as comparedwith the mixer-processing, the methane conversion rate was significantlyenhanced immediately after the start of the measurement. When themicro-processing was performed, as compared with the case where themixer-processing was performed, the final methane conversion rate wassatisfactory.

Then, the results of the measurements when the palm was used as theorganic substance are shown in Table 2, and the transition of themethane conversion rate in each of the samples is shown in FIG. 10. Inaddition, in a case where the palm was used as the organic substance,the measurements were performed only when the nano-processed productsand the micro-processed products in the Examples were used.

TABLE 2 Amount Input Input Input CH4max Sample Sludge Water of samplegCOD/TS COD (mg/l) TS (%) amount (g) amount (ml) COD (g) (ml)Micro-processed 1 30 g 9.0 g 1.0 ml — 130,600 — — 1.01 0.1319 50.39Micro-processed 2 — 130,600 — — 1.01 0.1319 50.39 Micro-processed 3 —130,600 — — 1.01 0.1319 50.39 Nano-processed 1 30 g 8.3 g 1.7 ml —78,067 — — 1.69 0.1319 50.40 Nano-processed 2 — 78,067 — — 1.69 0.131950.40 Nano-processed 3 — 78,067 — — 1.69 0.1319 50.40

When the palm was used as the organic substance, as shown in Table 2,methane gas was generated in each of the nano-processing and themicro-processing.

Also, as shown in FIG. 10, when the nano-processing was performed, themethane conversion rate was enhanced whereas when the micro-processingwas performed, the methane conversion rate was increased 10 days afterthe start of the measurement.

Then, the results of the measurements when the coffee grounds were usedas the organic substance are shown in Table 3, and the transition of themethane conversion rate in each of the samples is shown in FIG. 11.

TABLE 3 Amount Input Input Input CH4max Sample Sludge Water of samplegCOD/TS COD (mg/l) TS (%) amount (g) amount (ml) COD (g) (ml)Mixer-processed 1 30 g 9.5 g 0.5 g  — 270,500 — — 0.49 0.1325 50.64Mixer-processed 2 — 270,500 — — 0.49 0.1325 50.64 Mixer-processed 3 —270,500 — — 0.49 0.1325 50.64 Micro-processed 1 30 g 9.2 g 0.8 ml —174,267 — — 0.76 0.1324 50.60 Micro-processed 2 — 174,267 — — 0.760.1324 50.60 Micro-processed 3 — 174,267 — — 0.76 0.1324 50.60Nano-processed 1 30 g 8.4 g 1.6 ml — 82,267 — — 1.60 0.1316 50.29Nano-processed 2 — 82,267 — — 1.60 0.1316 50.29 Nano-processed 3 —82,267 — — 1.60 0.1316 50.29

When the coffee grounds were used as the organic substance, as shown inTable 3, methane gas was generated in each of the nano-processing, themicro-processing and the mixer-processing.

Also, as shown in FIG. 11, when the nano-processing was performed, ascompared with the case where the mixer-processing was performed, themethane conversion rate was significantly enhanced immediately after thestart of the measurement. When the micro-processing was performed, ascompared with the case where the mixer-processing was performed, themethane conversion rate was enhanced 5 days after the start of themeasurement.

The present invention is, for example, utilized for decomposing, with ananaerobic microorganism, an organic waste containing organic substancessuch as food residues, livestock manure and woody raw materials andthereby causing methane fermentation so as to generate useful methanegas.

The invention claimed is:
 1. A methane fermentation method comprisingthe steps of: a pulverization step of finely pulverizing an organicsubstance in which a moisture content is adjusted such that a solidcontent of the organic substance is equal to or less than 10% in a wetbead mill; a methane fermentation step of supplying the organicsubstance which is finely pulverized in the pulverization step to amethane fermentation chamber and decomposing the organic substance withan anaerobic microorganism to cause methane fermentation so as togenerate methane gas; a separation step of separating, with anultrafiltration (UF) membrane, a digestive fluid after the methanefermentation in the methane fermentation step into a concentrated liquidand a permeation liquid; a return step of returning the concentratedliquid separated in the separation step to the methane fermentationchamber; and an agitation step of agitating contents of the methanefermentation chamber with a water flow produced by the return of theconcentrated liquid in the return step.
 2. The methane fermentationmethod according to claim 1, wherein the methane fermentation step, theseparation step, the return step and the agitation step are repeated. 3.The methane fermentation method according to claim 1, wherein in thepulverization step, the organic substance is finely pulverized in thewet bead mill such that a particle diameter at 50% in a cumulativedistribution with respect to a volume is equal to or less than 20 μm. 4.The methane fermentation method according to claim 1, wherein in thepulverization step, the organic substance is finely pulverized in thewet bead mill for one hour or more.
 5. The methane fermentation methodaccording to claim 1, wherein in the separation step, the permeationliquid separated with the UF membrane is separated with a reverseosmosis (RO) membrane.